1. Field of the Invention
This document relates to a pixel circuit of an organic light emitting display.
2. Discussion of the Related Art
The importance of flat panel displays has recently increased with the growth of multimedia. Various types of flat panel displays such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting displays have been put to practical use.
The organic light emitting display has rapid response time, low power consumption, and self-emission structure. Furthermore, the organic light emitting display has a wide viewing angle, so that it can excellently display a moving picture regardless of the size of the screen or the position of a viewer. Because the organic light emitting display may be manufactured in low temperature environment using a semiconductor fabrication process, the organic light emitting display has a simple manufacturing process. Hence, the organic light emitting display is attractive as a next generation display.
Generally, the organic light emitting display has N×M organic light emitting diodes arranged in a matrix format and may be voltage driven or current driven, thereby displaying a predetermined image. The driving methods of the organic light emitting display include a passive matrix type and an active matrix type using a thin film transistor. In the passive matrix type, an anode electrode is at right angles to a cathode electrode. The anode electrode is selected by a scan signal and the cathode electrode receives a data signal, so that an organic light emitting diode (OLED) emits light in response to the data signal applied between the cathode electrode and the anode electrode. In the active matrix type, the thin film transistor is connected to a pixel electrode and a gate electrode of the thin film transistor is connected to a capacitor, so that the OLED emits light depending on a voltage stored in the capacitor.
FIG. 1 is a block diagram of a related art organic light emitting display.
Referring to FIG. 1, the organic light emitting display has a display panel 110, a scan driver 120, a data driver 130, a controller 140, and a power supply 150.
The display panel 110 includes data lines D1-Dm, scan lines S1-Sn, and pixel circuits P11-Pnm. The data lines D1-Dm are arranged in a first direction, and cross the scan lines S1-Sn arranged in a second direction. The pixel circuits P11-Pnm are disposed at pixel areas defined by the data lines D1-Dm and the scan lines S1-Sn.
The controller 140 outputs control signals to the scan driver 120, the data driver and the power supply 150. The power supply 150 outputs necessary voltages to the scan driver 120, the data driver and the display panel 110 in response to the control signals received from the controller 140.
The scan driver 120 outputs scan signals to the scan lines S1-Sn connected to the scan driver 120 in response to the control signal of the controller 140. Hence, the pixel circuits P11-Pnm of the display panel 110 are selected by the scan signals.
The data driver 130 outputs data signals synchronized with the scan signals to the data lines D1-Dm connected to the data driver 130 in response to the control signal of the controller 140. Then, the data driver 130 applies the data signals to the corresponding pixel circuits P11-Pnm through the data lines D1-Dm. Hence, the pixel circuits P11-Pnm emit light in response to the data signal to display a predetermined image on the display panel 110.
FIG. 2 is a circuit diagram of a pixel circuit of a related art organic light emitting display.
Referring to FIG. 2, the pixel circuit includes a switching transistor MS, a capacitor Cgs, a driving transistor MD, and an organic light emitting diode (OLED). The switching transistor MS transmits a data signal from a data line Dm in response to a scan signal of a scan line Sn. The data signal received through the switching transistor MS is stored in the capacitor Cgs. The data signal stored in the capacitor Cgs is used to generate a driving current for the driving transistor MD. Hence, the OLED emits light depending on the driving current.
A driving current IOLED flowing into the OLED is expressed by the following equation 1.
                                          I            OLED                    =                                    1              2                        ⁢                                          K                ⁡                                  (                                      Vgs                    -                    Vth                                    )                                            2                                      ,                                  ⁢                  K          =                      μ            ⁢                                                  ⁢            Cox            ⁢                          W              L                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Where μ denotes field-effect mobility, Cox capacitance of an insulating layer, W a channel width, and L a channel length.
The current flowing into the OLED of the pixel circuit may be determined by a gate voltage and a threshold voltage Vth of the driving transistor MD and a voltage of a first power supply VDD. To ensure uniformity between luminances of pixels, uniformity of characteristics of thin film transistors, particularly, uniformity of the threshold voltages and mobility of thin film transistors should be achieved.
However, because a voltage drop occurs in a first power supply line VDD for supplying power supply to each pixel circuit, a voltage of the first power supply line VDD supplied to each pixel circuit varies according to a location of each pixel circuit. Because there are changes in a current flowing into the OLED of each pixel circuit, non-uniformity between luminances of pixels occurs.
Further, a thin film transistor used in the organic light emitting display uses poly-silicon. Grain size of the poly-silicon may not be uniform. Hence, uniformity of characteristics of thin film transistors, particularly, uniformity of the threshold voltages and mobility of thin film transistors should be achieved in order to have a uniform display.